In high speed device applications of the microelectronic and telecommunication industries, II-VI and III-V compound semiconductor materials offer a number of advantages over devices based on silicon semiconductors. For instance, the high electron mobility of III-V substrates, such as Indium Phosphide (InP) or Indium Gallium Arsenide (INGaAs) are advantageous in the high speed active device structures used in optical fiber communication applications that include passive device components, such as metal-insulator-metal capacitors. Also, the wide band gap properties of II-VI and III-V semiconductor materials have high break-down voltages that make them useful in modulator driver applications in optoelectronic devices.
The broad application of II-VI and III-V compound semiconductors in such devices has been problematic, however. One of the problems encountered, for example, is poor adhesion between capacitor insulating layers, comprising high dielectric constant materials, such as silicon nitride, and conducting layers comprising inert metals, including noble metals, such as gold, palladium or platinum. In comparison, for silicon-based semiconductors, there is better adhesion between insulating layers, such as silicon oxide, and conducting layers comprising non-inert metals, such as aluminum or copper. On the other hand, noble metals are preferred because such metals do not readily diffuse into II-VI or III-V materials and damage the semiconductor structure.
Previously proposed solutions to improve adhesion between insulating layers and conducting layers comprising inert metals are not satisfactory. Consider, for example, a metal-insulator-metal capacitor where the insulator is a dielectric material, such as conventional silicon nitride (Si3N4), and the upper and lower metal are inert metals, such as a noble metals. Typically, to promote adhesion of the insulator to the inert metal via metal adhesion, thin layers of metal, such as titanium or chromium are deposited between the insulator and the inert metal layers. The use of titanium as an adhesion promoter is problematic, for example, because titanium is readily oxidizable. Oxidation typically occurs during the transfer of a structure having titanium as the adhesion promoter from the tool for depositing the metal to the tool for depositing the insulating layer. Moreover, titanium oxides are not easily removed, requiring transfer to a separate tool for removal thus interfering with forming capacitors using noble metals. Chromium is not a good candidate adhesion promoter because chromium can act as a n-type dopant that readily diffuses into III-V materials.
Silicon nitride is known to adhere well to inert metals when deposited at temperatures of 300° C. or higher. Many II-VI and III-V compound semiconductors, however, must be kept at temperatures of less than 300° C. to avoid dissociation of the integrated substrate comprising, layers grown by Molecular Beam Epitaxy, for example, metal contact layers, and overlaying components. At such low temperatures, however, silicon nitride does not deposit on inert metals in a manner that allows acceptable adhesion.
Accordingly, an objective of the invention is a process for adhering inert metals to insulating and semiconductor layers without encountering the above-mentioned difficulties.